It is already known how to carry out continuous upflow filtration of a suspension or an emulsion through a filter bed as seen for example in Austrian Patent Application A/128/72 and Swedish Patents SE 7602999-0 (U.S. Pat. Nos. 4,197,201 and 4,126,546) and SE 8104869-6 (U.S. Pat. No. 4,482,457).
In each of these three patents, the suspension or emulsion to be treated flows in an upward direction through a filter bed or particles in a container where, one or more means are arranged at the lower part of the filter bed for introduction of the suspension or emulsion to be filtered, one or more means are arranged above the filter bed for withdrawal of filtrate and to form a filtrate zone above the filter bed. Means is provided for the transport of dirty filter media to a device for washing the dirty filter media.
In A/128/72 a washer is placed above the filtrate zone and consists of a hydrocyclone to which the wash liquid is fed to a holder below the washer so a part of the wash liquid flows up through the hydrocyclone and will be withdrawn as reject water at the upper end of the hydrocyclone.
The washer in SE 8104869-6 (U.S. Pat. No. 4,482,457) is located above the filtrate zone and consists of an inclined and horizontal moving screw conveyor-type washer, where dirty filter media is fed to the bottom and wash liquid added to the top. By the rotation of the screw the dirty filter media will be transported up through the wash liquid so the filter media will be cleaned. Washed filter media will be fed from the upper end of the wash device and transported to the top of the filter bed and reject liquid will be withdrawn from the lower end of the wash device.
SE 7602999-0 (U.S. Pat. No. 4,197,201) has a washer located in the filtrate zone above the filter bed and consists of a counter current washer to which the dirty filter media is fed to the upper part of the wash device and flows downwards in counter current to the upward flow of a wash liquid fed at the lower part of the wash device and taken from the filtrate zone above the filter bed. The washed filter material is returned to the top of the filter bed and reject water is withdrawn from the top of the wash device.
In all of these three prior known designs, the dirty filter media has been washed and devices located above the filter bed which will result in the filter apparatus being relatively tall which presents a number of disadvantages.
U.S. Pat. No. 2,057,887 describes another design of a continuous upflow sand filter to which the suspension or emulsion to be filtered is fed to the filter bed by one or more means at the lower part of the filter bed and is withdrawn as filtrate by means at the upper part of the filter bed. The dirty filter media material is withdrawn by lift means at the bottom of the filter tank after which liquid and sediment is removed from the filter media material before cleaned media is returned to the top of the filter tank. This construction also results in a relatively tall apparatus. Furthermore, the construction includes a complicated design for feed of filter media material to the top of the filter tank.
The devices for transport and washing of dirty filter media material and return of washed filter media material to the top of the filter bed are important parts of a continuously operating sand filter. It is also important that these devices are reached easily for observation and control of all the different flows associated with the filter process. The type of filter media material normally used by these types of filters is sand with a grain size of 0.8-1.2 mm which is a highly abrasive material and thus moving parts should be avoided. To be sure that the dirty filter material will be effectively washed, washing should be done under controlled conditions.